The Effects of Various Factors on Plant Yield
Yield of commercially valuable species in the natural environment may be suboptimal as plants often grow under unfavorable conditions, such as at an inappropriate temperature or with a limited supply of soil nutrients, light, or water availability. For example, nitrogen (N) and phosphorus (P) are critical limiting nutrients for plants. Phosphorus is second only to nitrogen in its importance as a macronutrient for plant growth and to its impact on crop yield. Plants have evolved several strategies to help cope with P and N deprivation that include metabolic as well as developmental adaptations. Most, if not all, of these strategies have components that are regulated at the level of transcription and therefore are amenable to manipulation by transcription factors. Metabolic adaptations include increasing the availability of P and N by increasing uptake from the soil though the induction of high affinity and low affinity transporters, and/or increasing its mobilization in the plant. Developmental adaptations include increases in primary and secondary roots, increases in root hair number and length, and associations with mycorrhizal fungi (Bates and Lynch (1996); Harrison (1999)).
Nitrogen and carbon metabolism are tightly linked in almost every biochemical pathway in the plant. Carbon metabolites regulate genes involved in N acquisition and metabolism, and are known to affect germination and the expression of photosynthetic genes (Coruzzi et al. (2001)) and hence growth. Early studies on nitrate reductase (NR) in 1976 showed that NR activity could be affected by Glc/Suc (Crawford (1995); Daniel-Vedele et al. (1996)). Those observations were supported by later experiments that showed sugars induce NR mRNA in dark-adapted, green seedlings (Cheng et al. (1992)). C and N may have antagonistic relationships as signaling molecules; light induction of NR activity and mRNA levels can be mimicked by C metabolites and N-metabolites cause repression of NR induction in tobacco (Vincentz et al. (1992)). Gene regulation by C/N (carbon-nitrogen balance) status has been demonstrated for a number of N-metabolic genes (Stitt (1999)); Coruzzi et al. (2001)). Thus, a plant with altered C/N sensing may exhibit improved germination and/or growth under nitrogen-limiting conditions.
Water deficit is a major limitation of crop yields. In water-limited environments, crop yield is a function of water use, water use efficiency (WUE; defined as aerial biomass yield/water use) and the harvest index (HI; the ratio of yield biomass to the total cumulative biomass at harvest). WUE is a complex trait that involves water and CO2 uptake, transport and exchange at the leaf surface (transpiration). Improved WUE has been proposed as a criterion for yield improvement under drought. Water deficit can also have adverse effects in the form of increased susceptibility to disease and pests, reduced plant growth and reproductive failure. Useful genes for expression especially during water deficit are genes which promote aspects of plant growth or fertility, genes which impart disease resistance, genes which impart pest resistance, and the like. These limitations can delay growth and development, reduce productivity, and in extreme cases, cause the plant to die. Enhanced tolerance to these stresses would lead to yield increases in conventional varieties and reduce yield variation in hybrid varieties.
Another factor affecting yield is the number of plants that can be grown per acre. For crop species, planting or population density varies from a crop to a crop, from one growing region to another, and from year to year.
A plant's traits, including its biochemical, developmental, or phenotypic characteristics that enhance yield or tolerance to various abiotic stresses, may be controlled through a number of cellular processes. One important way to manipulate that control is through transcription factors—proteins that influence the expression of a particular gene or sets of genes. Transformed and transgenic plants that comprise cells having altered levels of at least one selected transcription factor, for example, possess advantageous or desirable traits. Strategies for manipulating traits by altering a plant cell's transcription factor content can therefore result in plants and crops with commercially valuable properties.